Recently, a long-distance optical transmission system using a Raman amplifier has been put to practical use as a system that realizes long distance transmission of wavelength division multiplexing (WDM) signal light. The Raman amplifier injects pump light from a reception side into an optical transmission line serving as an amplifying medium, and makes the pump light propagate in a direction opposite to a propagation direction of a signal light, to thereby amplify the signal light propagating on the optical transmission line by an effect of stimulated Raman scattering.
As one important function in the optical transmission system using the Raman amplifier, there is an optical output control function of the Raman amplifier. That is to say, in order to obtain a Raman gain in the optical transmission line, the Raman amplifier needs to inject pump light having an extremely high optical output power into the optical transmission line. Therefore, a safety measure for the pump light output from the Raman amplifier to the optical transmission line becomes important. Specifically, as the safety measure, since the power of the pump light output from the Raman amplifier to the optical transmission line at the time of normal operation becomes a high level of a level that may have a dangerous effect on a human body, it is required to reliably detect an abnormal state of the system (for example, when an output terminal of the Raman amplifier is disconnected), and stop the output of the pump light from the Raman amplifier, or decrease the output power of the pump light to a safe level.
As conventional technology for confirming whether the output terminal of the Raman amplifier and the optical transmission line are securely connected to each other, for example, in Japanese Laid-open Patent Publication No. 2002-252595 there is disclosed a method of determining a connection state of the output terminal of the Raman amplifier by confirming communication of an optical supervisory channel (OSC) transferred between optical transmission devices on the system.
Moreover, for example, in Japanese Laid-open Patent Publication No. 2002-221743, there is proposed a technique in which Raman pump light is modulated and transmitted to the optical transmission line, and a modulated component included in the light reflected at the output terminal of the Raman amplifier is detected to thereby confirm a connection state of the output terminal of the Raman amplifier without being affected by ambient light, and perform output control of the Raman pump light.
However, in the abovementioned conventional technology using the OSC, there is a problem in that if a length of the optical transmission line in one repeating section becomes long, connection confirmation by communication of the OSC becomes difficult.
This problem is explained in more detail. Generally, as an application of the Raman amplifier, long span Raman and long-haul Raman are known. The long span Raman is an application method for increasing the length of the transmission distance of one repeating section, in which a signal gain is provided by Raman amplification in the optical transmission line, so that a level of the signal light propagating on the optical transmission line and input to an optical transmission device on a downstream side exceeds a lower limit of an input level range of an erbium doped fiber amplifier (EDFA) provided inside the device. On the other hand, the long-haul Raman is an application method for increasing the number of repeaters (the number of spans) in a multi-stage repeating system, in which an input level of the signal light to the EDFA or the like is increased by Raman amplification in the optical transmission line, so that deterioration of an optical signal-to-noise ratio (OSNR) in the EDFA or the like at each repeating node is suppressed. In the case of the long-haul Raman, the power of the Raman pump light is optimized so that deterioration of the OSNR in the total optical amplification of the Raman amplifier and the EDFA is minimized.
Among the above applications of the Raman amplifier, in a system corresponding to the former long span Raman, if the transmission distance of one repeating section becomes long, transmission loss increases making it difficult to receive the OSC. Therefore it is necessary to take measures to Raman-amplify not only the signal light but also the OSC in the optical transmission line. In this case, since communication of the OSC is not confirmed without supply of the Raman pump light to the optical transmission line, the connection state between the output terminal of the Raman amplifier and the optical transmission line cannot be determined by using the OSC. In order to communicate the OSC without using the Raman amplification, the transmission power of the OSC needs to be increased or the OSC needs to be amplified on the reception side by an optical amplifying device other than the Raman amplifier. However this has a drawback in that, in a state with the Raman amplifier being activated in the normal operation, the device for increasing the reception power of the OSC is not required, and hence the cost therefor is an extra.
Moreover, in the optical output control of the Raman amplifier using the OSC, it is necessary to perform communication between an OSC unit in an existing system and a Raman amplifier unit to be added, at the time of upgrading an existing optical transmission system which does not use the Raman amplifier to a system which applies the Raman amplifier. Therefore, there is also a development problem in that initial design of the existing system must be performed on the assumption of expansion to the system which applies the Raman amplifier.
As for the aforementioned conventional technique for confirming the connection of the output terminal by applying modulation to the Raman pump light, for example, when the optical transmission line is cut at a position far from a receiving end (output terminal of the Raman amplifier), there is a possibility that high-level pump light may be output from the Raman amplifier, which is a problem. That is, in the conventional technique, basically the configuration is such that when the connection of the optical transmission line is disconnected, a modulated component of the reflected light that is reflected at the released end and returns to the Raman amplifier is detected, to thereby confirm the connection state of the optical transmission line, and control the optical output of the Raman amplifier. In this configuration, when the optical transmission line is cut at a position far from the receiving end, the reflected light on the cut surface is attenuated due to the transmission loss while the reflected light is returning to the receiving end, thereby making it difficult to detect the reflected light at the receiving end. Specifically, the power of Fresnel-reflected light at the cut surface becomes about −14 dB of the power of the light incident on the cut surface. If it is assumed that a distance from the cut surface to the receiving end is L [km], the output power of the modulated pump light is P [dBm], and a loss coefficient of the optical transmission line is α [dB/km], the power of the reflected light returning from the cut surface to the receiving end becomes P-14-2×L×α [dBm]. Assuming as above that the output power of the Raman pump light is set high so that the reflected light can be detected at the receiving end, the power already exceeds the safe level at the receiving end, and hence there is an inherent problem in using such pump light for confirming the connection of the optical transmission line.
In the case where reflected light from a cut surface cannot be detected at the receiving end, it is determined that the connection state of the optical transmission line is normal, and hence high-level pump light is output from the Raman amplifier. Assuming that the output power of the pump light at this time is Px, the power of the pump light output from the remote cut surface to the outside becomes Px-L×α. Because the output power Px of the pump light is set to a very high level in order to obtain a desired Raman gain, the power exceeds the safe level even at a great distance.
In the above cited document (No. 2002-221743) is shown a configuration in which modulated backward Raman pump light is observed with a device on an upstream side, and the level of the modulated component is detected to thereby confirm the connection state of a connector on the downstream side, and output control of the signal light from the upstream side and of the forward Raman pump light is performed. However in this configuration it is not confirmed on the downstream side that the modulated component of the backward Raman pump light has been detected on the upstream side, and the output control of the Raman amplifier on the downstream side is performed basically by detecting the reflected light of the backward Raman pump light. Therefore, when the optical transmission line is cut at a great distance from the receiving end, as in the above case, high-level backward Raman pump light is output from the Raman amplifier on the downstream side.